This present invention relates to contrast agent imaging. In particular, transmission techniques for ultrasound contrast agent imaging are provided.
Contrast agents are injected into a patient's tissue. Due to a difference in response between contrast agents and fluids or contrast agents and tissues, contrast agents may assist diagnosis, such as for the assessment and differentiation of viable myocardium from nonviable myocardium, detection and characterization of focal liver lesions, indeterminate renal lesion characterization, breast lesion characterization, transcraneal studies, early identification of prostate lesions, or other vascular visualization enhancements. Being able to separate contrast agents from tissue in ultrasound imaging may assist diagnosis. However, contrast agents are destroyed by high power ultrasound, and tissue and contrast agents may have similar responses at fundamental frequencies of lower power transmitted ultrasound.
Various techniques have been developed for low mechanical index or low energy ultrasound contrast agent imaging. In one approach, the difference in harmonic response of contrast agents from tissues provides better contrast to tissue ratio. Two or more waveforms are transmitted sequentially with different phases, such as opposite phasing. By adding the responses to each of the sequential transmissions, acoustic energy at fundamental frequencies is cancelled out and second harmonic energy remains. However, tissue does generate some second harmonic or other harmonic information, reducing the desired contrast tissue ratio. Various other techniques for transmitting two or more pulses sequentially with different phases and/or magnitudes and then combining responsive echoes with different weightings and with different functions to cancel tissue signals and detect contrast agents has been suggested. In order to achieve better suppression of tissue signals, echoes responsive to three or more sequential transmissions are combined, but at a sacrifice of frame rate.